I'm sure that lots of people will come on here and say that they have no problems with lightly padded bike bags,
but...
I put my new £2,000 Bianchi in one for its first trip to Spain. I was sitting in a seat above the baggage hold door, waiting to disembark from the plane, and suddenly saw my bike bag come flying out of the hatch and drop several metres onto a pile of suitcases on a luggage trailer.
When I looked at the bike at the hotel, one of the frame tubes had a dent and a crack in it. I may still have a photo somewhere... NOT IMPRESSED! Here you go (the black dots were marks I put on to see if the crack was spreading)...
After that I bought a reinforced bike box. That successfully protected the bike on my subsequent holidays, but baggage handlers threw that about on one trip and broke the corner of it! (I repaired the box with a piece of sheet aluminium cut and bent to fit and glued in with epoxy.)
I reckon something that obviously has a bike in it, but doesn't look too secure is probably the best bet.
Some advice:
- Unbolt the rear derailleur, wrap it in bubble wrap, and tie it up inside the rear dropouts. A Scottish lad on my holiday had his derailleur hanger snapped off by his baggage mishandlers!
- Get hold of spacers to put between the fork dropouts and between the frame dropouts. The ones I had were plastic rods which I had to tap into place. They reduce the risk of the fork or frame getting crush damage.
- Consider putting pipe lagging on the frame tubes and fork legs for extra protection